191 1.] THE MAMMALIAN OVARY. 233 



in the cortex of the ovaries at a place where folHcles He normally 

 and are found within follicle-like cavities, they can only be derived 

 from ova developing parthenogenetically. Fertilization through sper- 

 matozoa can be excluded, inasmuch as the history of some of these 

 animals is known to us and precludes such an interpretation. It is 

 very probable that the parthenogenetic development sets in soon after 

 ovulation, the altered conditions in the ovaries at that time (varia- 

 tions in blood pressure, in intrafollicular pressure or changes in gas 

 exchange) supplying the necessary stimulus. This interpretation 

 agrees well with ni}- former observations concerning the parallelism 

 existing between the first segmentations taking place in non-fertilized 

 ova within the ovary and certain stages of atresia of follicles. - 



It is also of interest to note that frequently these changes are 

 multiple, several ova undergoing parthenogenetic development in the 

 same ovary. 



We may, therefore, conclude that in at least ten per cent, of all 

 guinea-pigs parthenogenetic development of the ova within the ovary 

 starts at some period of the life of the animal. The later stages of 

 these developing" embryos bear some resemblance to chorionepithe- 

 liomata, certain tumor-like formations consisting of proliferating 

 chorion tissue. During ovulation these structures are occasionallv 

 injured by hemorrhages and they are ultimately invaded and sup- 

 planted by the neighboring connective tissues. 



These observations throw furthermore light on certain interesting 

 tumors that are especially found in the ovaries and testicles, namely : 

 the teratoid tumors and the chorionepitheliomata. My observations 

 are a strong argument in favor of the view that teratoid tumors that 

 are found in the ovaries arc not derived from misplaced blastomeres, 

 as Bonnet and Marchand believed, but that the older view is correct 

 according to which they are derived from parthenogenetically devel- 

 oped ova, an opinion which I, also, expressed on previous occasions. 

 The same statement can be made in the case of the chorionepithe- 

 liomata that occur in the ovaries and in the testicles. I believe that 

 the observations here recorded clear up the mechanism of the sexual 



^ " On Progressive Changes in the Ova in Mammalian Ovaries," Jourual 

 of Medical Research, Vol. i, 1901. 



